It is common practice in the metallurgical art to utilize injection plug devices to distribute inert gas into molten metal contained in a ladle. Typically, the ladle has a refractory block inwardly adjacent its bottom surface with a central opening in alignment with an opening in the ladle bottom to accommodate insertable registration of a gas injection plug. Such devices are either permeable refractory elements which permit a gas flow to filter through the plug body or solid impervious refractory elements having an array of capillary passages extending through the plug body to conduct a series of discrete gas streams therethrough. U.S. Pat. No. 3,330,645 discloses both the foregoing concepts in one device, utilizing a porous plug material and also providing tubular passageways through the porous body.
Utilization of a solid impervious refractory plug with provision of a peripheral or annular gas flow passage, between the outer surface of the plug and a surrounding metallic sleeve, has been found to have certain distinct advantages. A solid plug has much greater strength and erosion resistance as compared to a porous plug, and the maintenance of continuous open flow conditions is greatly improved. U.S. Pat. Nos. 4,396,179 and 4,538,795 disclose plug structures which utilize a solid refractory core within a surrounding shell spaced from the outer surface of the core to provide a concentric passageway for delivering gas into the molten metal within a ladle, and the advantages of such structures are explained in these patents. A variation of the foregoing concept is found in U.S. Pat. No. 4,462,576 which teaches the provision of a fibrous permeable ceramic layer concentrically located between the outer surface of a frusto-conical solid refractory plug and a surrounding metal jacket.
Another development is the provision of a gas injection plug device wherein channels are preformed in the lateral surface of a solid refractory plug and a properly sized metallic sleeve or canister is positioned about the plug in a heated condition and allowed to cool and shrink tightly to the plug body, thereby closing over the plug surface channels to form an array of gas distribution passages. Provision of the longitudinally-extending slots in the lateral surface of the plug and shrinking the metallic sleeve tightly on the plug body assures a unitized structure, however, there has been a problem in the formation of such devices and maintaining a smooth symmetrical exterior configuration because of the tendency of the thin-walled metal shell to deform inwardly toward the plug slots and thus create a wavy and uneven shell outer surface which will not register smoothly and contiguously within the opening or socket provided in the refractory block of the ladle. Moreover, blockage of the outlet ends of the passages by molten metal, although less of a problem than with permeable or porous plug bodies, nevertheless occurs in these devices.
Yet another attempt at resolving the problems of the prior art is disclosed in U.S. Pat. No. 4,938,461, which relates to an impervious refractory plug clad in a metal canister which is provided with an array of narrow peripheral passages formed in the lateral surface of the plug. These passages are defined by slots which are pre-formed in the peripheral surface of the plug as well as in the inner surface of the tightly fitting canister. An elongated metal strip, preferably of stainless steel is located in each passage to constitute heat sink elements which act to induce solidification of any molten metal which may tend to flow into the output ends of the passages when the gas is not flowing.
Despite these improvements, there still remains a need for ceramic plugs which are capable of providing gas to a molten metal vessel without experiencing plugging by solidified metal, and which can be made available in a variety of shapes and sizes to fit the particular end uses. In addition, a simplified method for making such ceramic plugs would be desirable.